1. Field of the Invention
The present invention relates to an electric power steering system that drives or controls a motor on the basis of a steering wheel torque exerted by a driver of a vehicle, and alleviates the driver's steering wheel torque by utilizing a power generated by the motor.
2. Description of the Related Art
Electric power steering systems exert the capability of an electric power steering system by driving a motor. Along with driving of the motor, an engine control unit (hereinafter called an ECU) that controls the motor or the like, or the motor dissipates heat. In order to prevent the ECU and motor from being broken due to heat dissipation, the electric power steering system generally includes an overheat protection unit that limits a current, which is conducted to the motor, so as to suppress the heat dissipation.
As the overheat protection unit included in the electric power steering system, a unit that estimates a temperature rise, which stems from heat dissipation from the heat dissipating regions of the ECU and motor respectively, on the basis of a cumulative value of the current conducted to the motor, computes a time, which it takes to turn off the power supply of a control unit on the basis of the estimated temperature rise, after discontinuing feed of power to the motor, and turns off the power supply of the control unit after the elapse of the computed time (refer to, for example, Patent Document 1, and a unit that estimates the heat dissipating states of the heat dissipating regions of the ECU and motor respectively using temperature sensors attached to the ECU and motor respectively, and discontinues a power supply relay on the basis of the estimated temperatures (refer to, for example, Patent Document 2 have been proposed in the past.    Patent Document 1: Japanese Patent No. 2892899 (JP-A-6-247324))    Patent Document 2: JP-A-2006-224690)
The foregoing existing overheat protection unit decides the heat dissipating states on the basis of a cumulative value of a current conducted to the motor or the values provided by the temperature sensors, and prevents the ECU and motor from being broken due to heat dissipation. If an error occurs in the computation performed to decide the heat dissipating states, there is a possibility that the overheat protection unit may not function and the ECU or motor may be damaged.
For example, since the existing overheat protection unit described in the Patent Document 1 decides the heat dissipating state on the basis of the cumulative value of the current conducted to the motor, and limits or discontinues the current to be conducted to the motor, if the cumulative value deviates from a true value because of occurrence of an abnormality in computation of an overheat protection limit value or in a RAM in which the cumulative value is stored, the succeeding computation is performed based on the erroneous cumulative value. As a result, limitation of the motor current to be performed by the overheat protection unit may be performed insufficiently or excessively.
As a unit that senses an error in computation to be performed in order to decide the heat dissipating state, a unit that allows a physically independent arithmetic unit or the like to perform the same calculation and monitors the unit or is monitored by the unit through communication is conceivable. However, this poses a problem in that the system becomes expensive.